Republican leaders in the House of Representatives are planning a vote on their health care bill on Thursday. It's been over a month since they abruptly pulled the original version of that bill from the House floor, which was a stark and early defeat for President Trump on one of his major campaign promises. NPR congressional correspondent Susan Davis joins us now from the Capitol. Hello, Sue.

SUSAN DAVIS, BYLINE: Hey, Robert.

SIEGEL: Speaker Paul Ryan has said that the Republicans wouldn't bring this bill back until they had enough votes to pass it. Does that mean they have enough votes to pass it?

DAVIS: They believe they do. House Majority Leader Kevin McCarthy just a few moments ago told us that he believes they have the vote, and they will - they have the votes to pass it. And the vote will happen early tomorrow afternoon. It could be very close. They can only lose about 22 Republicans and still pass the bill because we know every Democrat's going to go - going to vote against it.

And as of this evening, there had already been about 18 or 19 Republicans who were publicly against it and unexpected to change their positions. So that's a really narrow margin, and it could be one of the toughest votes Republicans have faced since they won the majority back in 2010.

SIEGEL: What changes have been made to this bill in order to get more support for it?

DAVIS: The final push was an amendment by Fred Upton. He's a Republican from Michigan, and he has a fair amount of health care expertise. He's a former chairman of the Energy and Commerce Committee. And he was initially a no, and he got to yes because they've included a proposal of his that would add in another $8 billion in funding for states to create these things that are called high-risk pools to cover sick people.

There's a lot we don't know about how this proposal would specifically work. And in the past, high-risk pools have been very expensive, and they've not done a very good job of providing coverage for sick people. We also don't know the economic impact of this bill at all. The House is going to vote on it without an official score from the nonpartisan Congressional Budget Office which weighs in on how much it's going to cost and how many people it's going to cover. So there's a lot of questions we still have about the bill this evening, but we do know the vote will happen tomorrow.

SIEGEL: Sue, when the House Republicans walked away from the earlier version of the health care bill in March, Speaker Paul Ryan said that Obamacare was the law of the land for the foreseeable future. What kind of political pressure did they face to turn this around?

DAVIS: You know, that was part of the argument for this renewed push of it all - is it was the reminder that this was something that Republicans had campaigned on for the better part of the past decade. And there was increasingly a view that failing to not even have a vote on it really risked sort of depressing the party's base. And there's - one of the things they're already talking about inside this building is the 2018 midterms and the need to keep the Republican base voter excited.

And I would also say that the president really engaged. In the - in this week, the president, Trump, and Vice President Mike Pence really decided they wanted this vote this week. The president was personally engaged in this. He was working the phones today. And they wanted a vote this week.

SIEGEL: Of course even if the House does approve this, Obamacare remains the law of the land because this bill would have to go to the Senate.

DAVIS: Right.

SIEGEL: What prospects would this have there?

DAVIS: Right. It's always important remember that this is just one step of a process. They're passing a bill tomorrow, not the law. And there is something in here for every senator to dislike. The House proposal is going to land with a bit of a thud on the Senate side. There's conservative opposition to it over there. There's a tremendous amount of skepticism among moderate senators like Susan Collins of Maine and Lisa Murkowski of Alaska.

Senate Majority Leader Mitch McConnell is going to face the same tricky balance that Speaker Paul Ryan did of trying to find a coalition of conservative and moderate Republicans who can vote for something. And we know every Democrat in the Senate's going to vote against it. And Mitch McConnell has an even slimmer margin of error than the speaker does. He can only lose two Republicans and still pass a bill through the Senate.

SIEGEL: So even after tomorrow, we will not have heard the last of the debate over Obamacare and its future. That's...

DAVIS: No, there's a long way to go.

SIEGEL: ...NPR's Susan Davis with the news that House Republicans plan to vote on their health care bill tomorrow. Sue, thanks.